This invention relates to hydrophobe associative compositions that increase the viscosity of aqueous media containing water-soluble electrolytes.
As taught in Encyclopedia of Polymer Science and Technology, Interscience Publishers, Vol. I, 192 (1964), and Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Ed., Vol. 20, pp. 207-230 (1982), John Wiley & Sons (both of which are incorporated herein by reference), it is known that the viscosity of an aqueous medium may be increased by the addition of a water-soluble polymer. Such water-soluble polymers include polyacrylamide, acrylamide/acrylic acid copolymer, sodium polyacrylate, carboxymethyl cellulose, hydroxyethyl cellulose, methyl cellulose, polysaccharide as well as naturally occurring gums such as guar gum and chemically modified gums such as hydroxypropyl guar gum. As a result of this thickening capability, there are many existing, as well as potential, industrial applications for aqueous media thickened with such water-swellable polymers.
Unfortunately, however, the aforementioned conventional water-soluble polymers suffer from deficiencies or limitations in actual use in such industrial applications. For example, for reasons of efficiency and economical considerations, it is common to employ very high molecular weight versions of such polymers. However, during many industrial applications, the conditions present during the practice of the application involve exposing an aqueous medium containing the high molecular weight water-soluble polymer to high shear. Such shear often causes mechanical degradation of the polymer and thus reduces the viscosity of the aqueous medium. While lower molecular weight polymers are less sensitive to shear degradation, they must be used in much higher concentrations in order to achieve the desired level of visosity.
Secondly, while ionic water-soluble polymers such as neutralized acrylamide/acrylic acid copolymer, sodium polyacrylate, polystyrene sulfonate and the like are more efficient thickeners in deionized water than their nonionic counterparts, their thickening ability is greatly reduced by the presence of water-soluble electrolytes such as sodium chloride, calcium chloride and magnesium sulfate in the aqueous medium. Such electrolytes are present in the aqueous media employed in most industrial applications, particularly those requiring the use of ground waters in subterranean formations as in enhanced oil recovery.
Finally, in many applications, the aqueous medium thickened with water-soluble polymer is exposed to temperatures in the range of 30.degree. C. to 100.degree. C. which normally causes reduction of viscosity. Such high temperatures are particularly common in enhanced oil recovery applications wherein the aqueous medium is pumped underground to depths of 5000 to 20,000 feet, as is common for mobility control fluids and packing fluids.
In attempts to overcome some of the aforementioned deficiencies of the conventional water-soluble polymers, it has been a common practice to cross-link the polymer in order to improve resistance to thermal as well as shear degradation. See, for example, U.S. Pat. No. 3,247,171. Such attempts have generally not been successful.
More recently, as taught in U.S. Pat. No. 3,984,333, an aqueous medium has been thickened by dissolving a block copolymer having water-soluble blocks and water-insoluble blocks in the aqueous medium. While such block copolymers apparently exhibit reasonably good resistance to shear degradation, such polymers are difficult and often impractical to prepare. More importantly, such polymers do not exhibit significant tolerance of electrolytes normally present in the aqueous media to be thickened.
While the cellulosic derivatives such as hydroxyethyl cellulose and biopolymers exhibit acceptable tolerance to the presence of electrolytes, cellulosic derivatives are generally ineffective at the low concentrations that are economical and exhibit poor thermal stability. The biopolymers such as xanthan gums exhibit acceptable thermal stability, resistance to shear degradation and electrolyte tolerance. Unfortunately, such biopolymers are generally very expensive and are susceptible to biodegradation.
U.S. Pat. No. 4,463,152 discloses a copolymer of acrylamide and a nonionic surfactant monomer such as: ##STR4## wherein R is hydrophobic and n equals 1 to 60. The patent claims that these copolymers are more efficient viscosification agents than homopolymers of acrylamide and are different from hydrolyzed polyacrylamides (a copolymer of acrylamide and acrylic acid) by virtue of higher thickening ability in a concentration dependence of viscosity in a brine environment and often higher retention of viscosity in the presence of inorganic electrolytes. The patentees state that the small amount of hydrophobic or water-insoluble alkyl groups on the surfactant comonomer is believed to result in intermolecular aggregation or interaction in solution, resulting in enhanced viscosity relative to a noninteracting polymer such as a polyacrylamide. Such polymers may be limited in their tolerance of electrolyte. While the viscosity of the thickened medium increases with salt concentration to a point above a certain level of salt concentration, the aqueous medium viscosity drops off sharply, thus such polymers have limited usefulness.
U.S. Pat. No. 4,432,881 discloses a thickened aqueous medium containing a water-soluble polymer having pendant hydrophobic groups, for example, a copolymer of acrylamide and dodecyl acrylate, said copolymer being combined with a water-dispersible surfactant. The copolymer formed also suffers from lowering of viscosity as the electrolyte concentration increases beyond a certain point because gel particles are formed which do not thicken the bulk media.
In view of the aforementioned deficiencies of conventional water-soluble polymers, it is highly desirable to provide a relatively inexpensive polymer composition capable of increasing or maintaining the viscosity of aqueous media containing electrolytes. It is further desirable that said polymer exhibits thermal stability, electrolyte tolerance and good resistance to shear and biological degradation. It is even further more desirable that aqueous media thickened with said water-soluble polymers are thickened over a broad range of electrolyte concentration.